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TCI Programmable Digital Ignition

Differences between CDI and TCI

CDI

CDI is fired by using a charged capacitor (typical voltages 300-600 v, typical capacity: 1-4 uf

Delay time is applied directly after the pickup pulse to fix the spark position

CDI spark is shorter and more powerful than TCI spark

CDI is more suitable to work at high rpm

CDI needs an inverter to increase voltage upto 300 or 400 volt if used with 12 volt battery

TCI

TCI is fired charging the coil itself before fire the spark. Time taken to charge the coil is called dwell time. The spark is made when the current across the coil stops, and the coil collapsates and fires the spark

Delay time is applied after the pickup pulse but dwell time has to be substract from the total time, to fix the spark position

TCI uses a simpler design and doesn't need capacitors

Dwell time in degrees (dwell time=1ms)

Dwell time is constant for each coil, typical values go from 2.5ms to 1ms. Because dwell time is a constant in seconds, number of degrees needed for dwell time varies for the different rpm values of engine (see graph). This make things a bit more complex than on CDI ignition, because the program needs to calculate where to start coil's charge (before than on CDI) to ensure the right dwell time even at high rpm.

How to calculate where should start the coil's charge:
at low rpm Tc will be after the ref. pulse: Tcl = dwell time - delay (table) (Tcl is negative at high rpm)
at high rpm Tc will be before the ref. pulse: Tch = period - dwell time + delay (table) (Tch = Tcl + period)

    TCI Schematic (PIC16F84) (current one)

Click here for the source code (tested on 16F84)

Notes about the code:

• This release of the program is a simplification of TCI, but it works for certain coils.
• TCI could be more complex for high speed coils: it would need to start the coil's charge after the reference pulse when engine is running at low rpm, and before the reference pulse when engine is running at high rpm. Current version only does it before ref pulse.
• This version always calculates the dwell time before the ref. pulse, but for low rpm dwell time will be longer than needed. This spents some extra current on the coil at low rpm. But if coils with certain resistance (3 ohm or higher) it won't be an issue, these coils accept wide dwell time range. Please don't test it on CDI coils or on-spark coils because its lower impedance.
• For example: with a dwell time of 1ms, and a spark advance of 30º, dwell time will start before the ref. pulse when higher than 5000 rpm, and after the pulse when lower than 5000 rpm.
• For low ohm coils, dwell time should be calculated with the highest accuracy possible (in future version) because we are using a schematic based on a IGBT and this device doesn't limit the coil's current (as on old transistor based circuits), and this may be dangerous for a low ohm coil.

    TCI Schematic (PIC16F873) (future)

Last modified: SEP/28/2007